In the application of glue and like viscous adhesives to the back of a stack of sheets or leaves or signatures in pad, book or folio binding, it is a common practice to use a roller-type adhesive applicator in which the roller is rotated in a vessel constituting a reservoir for the adhesive and has its upper portion lying out of the bath and disposed to contact the underside of the stack which can be passed over the roller, as a workpiece, with the sheets or leaves in an upright position, i.e. on edge. The adhesive or glue is thus picked up by the periphery of the roller from the bath and transferred to the back of the stack as the latter is moved across the roller and the roller is rotated.
In general, one or more doctor blades may co-operate with the roller to control the thickness of the adhesive layer on the latter before it contacts the stack.
In earlier approaches utilizing this system, the roller was generally rotated in the direction of movement of the stack, i.e. its periphery, and the stack moved in the same direction at the region of contact.
The stack, generally gripped in a clamp device and guided by rails or the like along a linear or curved path, crossed the roller at the top thereof. The uniformity of the layer and the efficiency of transfer of the adhesive left much to be desired and subsequent systems utilized an opposite movement of the roller and the stack. In other words, as the stack was moved continuously in one direction across the top of the roller, the periphery of the roller was driven in the opposite direction at the top thereof.
This technique was found to improve the adhesive-transfer efficiency and the uniformity of the coating.
However, as the leading edge of the stack approaching or contacted the roller with the opposite movement described above of the roller and the stack, a glob of the adhesive tended to form on the leading edge, thereby destroying the uniformity of the coating and posing the danger that the adhesive would drip from the stack and create problems.
To avoid this difficulty, it has been proposed to suspend the adhesive-applying roller and the adhesive reservoir and to provide means for dropping the top of the roller by a distance approximately equal to the thickness of the adhesive layer on the roller at the moment the leading edge would otherwise contact the roller, thereafter raising the system to bring about the contact and application of the adhesive. This system was successful in preventing the formation of a glob of adhesive at the leading edge of the stack but involved the use of complex kinematics, movable adhesive-applying assemblies, as well as careful adjustment of the positions and movements of the latter. The system frequently went out of adjustment and was relatively expensive and often unreliable, especially where the mass which had to be moved was relatively large.